1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor (also referred to below as a “photoreceptor”), a method of producing same, and an electrophotographic apparatus. The present invention more particularly relates to an electrophotographic photoreceptor that is principally formed of a conductive substrate and an organic material-containing photosensitive layer and that is used in, for example, electrophotography-based printers, copiers, and fax machines, and to a method of producing this electrophotographic photoreceptor and to an electrophotographic apparatus.
2. Background of the Related Art
The basic structure of an electrophotographic photoreceptor is a structure in which a photosensitive layer having a photoconductive function is disposed over a conductive substrate. Organic electrophotographic photoreceptors, which use organic compounds as the functional components responsible for charge generation and transport, have in recent years been the subject of active research and development based on advantages such as diversity of materials, high productivity, and safety, and their application to, for example, copiers and printers is moving forward.
The following functions are generally required in a photoreceptor: the ability to maintain a surface charge in the dark, the ability to generate charge upon exposure to light, and the ability to transport the generated charge. Photoreceptors include so-called monolayer photoreceptors, which are provided with a monolayer photosensitive layer in which these functions are combined, and so-called stacked (functionally separated) photoreceptors, which are provided with a photosensitive layer that has a stack of layers that are functionally separated primarily into a charge generation layer responsible for the function of charge generation upon exposure to light and a charge transport layer responsible for the function of maintaining a surface charge in the dark and the function of transporting the charge generated by the charge generation layer upon exposure to light.
The photosensitive layer is generally formed by the application onto a conductive substrate of a coating liquid provided by the dissolution or dispersion in an organic solvent of a charge generation material, a charge transport material, and a resin binder. Particularly in the case of the layer that is surfacemost in an organic photoreceptor, the use of polycarbonate, which exhibits an excellent flexibility and a good transparency to photoexposure and is resistant to the friction produced with the paper or toner-removal blade, as the organic binder, is frequently seen. Among the polycarbonates, the bisphenol Z-based polycarbonates are in wide use as the resin binder. The art in which such a polycarbonate is used as the resin binder is taught in, for example, Japanese Patent Application Laid-open No. S61-62040 (Patent Document 1).
So-called digital devices have become the mainstream for electrophotographic devices in recent years. With these digital devices, monochromatic light from, for example, an argon, helium-neon, or semiconductor laser or a light-emitting diode, is used as the exposure light source; the information, e.g., an image or characters, is digitized and converted to an optical signal; an electrostatic latent image is formed in the surface of the photoreceptor by irradiating the light onto the charged photoreceptor; and this electrostatic latent image is then visualized with toner.
Methods for charging the photoreceptor include noncontact charging methods, in which there is no contact between the photoreceptor and the charging member, e.g., a scorotron, and contact charging methods, in which the photoreceptor comes into contact with a charging member composed of a semiconducting rubber roller or brush. Compared with noncontact charging methods, contact charging methods offer the advantages, due to the occurrence of corona discharge in the immediate vicinity of the photoreceptor, of little ozone generation and enabling a reduction in the applied voltage. Thus, they have become the mainstream in particular in medium- to small-scale devices because they enable the realization of more compact and low-cost electrophotographic devices that generate little environmental pollution.
Wiping with a blade and a simultaneous development/cleaning process are the most commonly used means for cleaning the surface of the photoreceptor. In the case of a blade-based cleaning process, the residual untransferred toner on the photoreceptor surface is wiped off with a blade and recovered in a waste toner recovery box or returned to the developing unit. When such a cleaner based on wiping with a blade is employed, space is then required for the toner recovery box or for toner recycle and the fill status of the recovery box must also be monitored. In addition, when paper dust or external additives accumulate on the blade, scratches may be produced on the surface of the photoreceptor and the life of the photoreceptor may then be shortened. Processes have therefore been implemented in which the toner is recovered at the developing process or in which the residual toner adhered to the photoreceptor surface is magnetically or electrically lifted off immediately before the developing process.
Moreover, when a cleaning blade is used, the hardness of the blade and/or the contact pressure must be raised in order to improve the cleaning performance. This promotes wear in the photoreceptor surface, which can produce fluctuations in the potential and sensitivity and can thereby produce image abnormalities and, in the case of color devices, can produce problems with the color balance and reproducibility.
Methods for improving the surfacemost layer of the photoreceptor have been proposed in order to solve these problems. For example, methods are proposed in Japanese Patent Application Laid-open No. H1-205171 (Patent Document 2) and in Japanese Patent Application Laid-open No. H7-333881 (Patent Document 3) in which a filler is added to the surface layer of the photoreceptor in order to improve the durability of the photoreceptor surface. However, it is quite difficult with the methods for dispersing a filler in such a layer to bring about a uniform dispersion of the filler. Moreover, charge transport and charge generation become nonuniform due to the presence of aggregates of the filler, a lower transmissivity for the layer, and scattering of the exposure light by the filler, and this creates the risk of a reduction in the image properties. The addition of a dispersant is one method for improving the dispersibility of the filler, but in this case the dispersant itself influences the properties of the photoreceptor and as a consequence it has been difficult to bring about a balance between filler dispersibility and the properties of the photoreceptor.
In order to solve these problems, Japanese Patent Application Laid-open No. 2002-128883 (Patent Document 4) teaches a method in which the wear resistance is improved through the use of a resin having a siloxane structure introduced in the terminal structure. In Japanese Patent Application Laid-open No. 2005-115091 (Patent Document 5), the use of a polyarylate as a resin binder for the photosensitive layer is proposed and extensive and diverse investigations are carried out with the goal of, for example, improving the durability and mechanical strength. Japanese Patent Application Laid-open No. 2002-214807 (Patent Document 6) teaches a photoreceptor in which the following are used in the photosensitive layer: a polycarbonate resin and a polyarylate resin that has a siloxane structure obtained using a phenol-modified polysiloxane as the siloxane component. Japanese Patent Application Laid-open No. 2004-93865 (Patent Document 7) proposes an electrophotographic apparatus that is provided with a photoreceptor that contains a polyarylate resin structural unit. Japanese Patent Application Laid-open No. 2007-121751 (Patent Document 8) and Japanese Patent Application Laid-open No. 2010-96929 (Patent Document 9) propose a photoreceptor that uses a polyarylate resin in the photosensitive layer.
On the other hand, methods have been proposed in which a surface protective layer is formed on the photosensitive layer for the purpose of, for example, protecting the photosensitive layer, improving the mechanical strength, and improving the surface lubricity. However, the following problems have occurred with methods in which a surface-protective layer is formed: film formation on the charge transport layer has been problematic, and it has been difficult to bring about a satisfactory balance between the charge transport performance and the charge maintenance function.
As indicated above, the art disclosed in these patent documents cannot provide good maintenance of the electrical properties and/or image properties while at the same time achieving a satisfactory reduction in the amount of wear of the photoreceptor surface.
Therefore, an object of the present invention is to provide an electrophotographic photoreceptor that can provide additional reductions in the amount of surface wear while producing an excellent image. Further objects of the present invention are to provide a method of producing this electrophotographic photoreceptor and to provide an electrophotographic apparatus.